Nanowire preparation methods, compositions, and articles

ABSTRACT

Methods of producing metal nanowires employing tubular continuous-flow reactors and their products are described and claimed. Such methods can provide superior nanowire uniformity without agglomeration. Such nanowires are useful for electronic applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/347,986, filed Jan. 11, 2012, which claimed the benefit of U.S.Provisional Application No. 61/442,874, filed Feb. 15, 2011, entitledNANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, which ishereby incorporated by reference in its entirety.

SUMMARY

At least a first embodiment provides methods comprising feeding at leastone first composition comprising at least one first reducible metal ionto the contents of at least one continuous-flow reactor comprising atleast one tubular reactor; reducing the at least one reducible metal ionto at least one metal nanowire; and withdrawing at least one secondcomposition comprising the at least one metal nanowire from the contentsof the at least one continuous-flow reactor. In at least someembodiments, at least some of the withdrawing of the at least one secondcomposition occurs before at least some of the feeding of the at leastone first composition, or it occurs simultaneously with at least some ofthe feeding of the at least one first composition, or both. In somecases, the contents of the tubular reactor are not mixed with a rotatingagitator. The at least one continuous-flow reactor may optionallyconsist essentially of the at least one tubular reactor.

In at least some embodiments, the at least one first composition furthercomprises at least one polyol and at least one of a protecting agent, apolar polymer, or a polar copolymer. In some cases, all of thecomponents to be fed to the at least one continuous reactor may, forexample, be combined to form a single feed composition.

In some cases, the at least one first reducible metal ion may compriseat least one coinage metal ion, at least one ion from IUPAC Group 11, orat least one ion of silver. In at least some embodiments, the reductionmay be performed in the presence of at least one second ion or atomcomprising at least one ion or atom from IUPAC Group 8, at least one ionor atom from IUPAC Group 14, at least one iron ion or atom, or at leastone tin ion or atom. In some cases, the reduction may be performed inthe presence of a halide ion, such as, for example, a bromide ion, achloride ion, or an iodide ion, or the reduction may, in some cases, beperformed in the presence of a chloride ion.

At least some embodiments provide the metal nanowires produced accordingto such methods. The metal nanowires produced according to such methodsmay, for example, comprise a length of at least about 10 μm, or fromabout 10 μm to about 50 μm, or of approximately 20 μm.

At least some other embodiments provide one or more articles comprisingat least one such nanowire. Such articles may, for example, compriseelectronic devices, transparent conductive films, and the like.

At least a second embodiment provides methods comprising providing atleast one first composition comprising at least one first reduciblemetal ion, and reducing the at least one first reducible metal ion to atleast one first metal in the presence of at least one first protectingagent and at least one first solvent, where the reduction is performedin at least one first continuous-flow reactor comprising at least onetubular reactor. In at least some embodiments, the at least one firstreducible metal ion comprises at least one coinage metal ion, or atleast one ion from IUPAC Group 11, or at least one ion of silver. Insome cases, the at least one first compound comprises silver nitrate. Inat least some embodiments, the reduction may be carried out in thepresence of at least one element from IUPAC Group 8, such as, forexample, iron or an ion of iron, or in the presence of at least oneelement from IUPAC Group 14, such as, for example, tin or an ion of tin,or in the presence of at least one metal salt, such as, for example, atleast one metal chloride. In at least some embodiments, the at least onefirst protecting agent comprises at least one of one or moresurfactants, one or more acids, or one or more polar solvents, or itmay, for example, comprise polyvinylpyrrolidinone. In at least somecases, the at least one first solvent comprises at least one polyol,such as, for example, one or more of ethylene glycol, propylene glycol,glycerol, one or more sugars, or one or more carbohydrates. In at leastsome embodiments, the composition has a ratio of the total moles of theat least one second metal or metal ion to the moles of the at least onefirst reducible metal ion from about 0.0001 to about 0.1. The reductionmay be carried out at one or more temperatures, such as, for example,from about 80° C. to about 190° C. In at least some embodiments, thesecond composition comprises at least one coinage metal or coinage metalion, or at least one element from IUPAC Group 11, such as, for example,silver or an ion of silver.

At least some embodiments provide such methods, where the reduction iscarried out in the presence of at least one second compositioncomprising seed particles. The at least one second composition maycomprise at least one coinage metal or coinage metal ion, or at leastone element from IUPAC Group 11, such as, for example, silver or an ionof silver. In at least some embodiments, the seed particles are formedby a method comprising providing at least one third metal ion andcontacting the at least one third metal ion with at least one secondprotecting agent and at least one second solvent. Such a method may, forexample, be carried out in at least one second continuous-flow reactor,which may, for example, comprise at least one tubular reactor.

Other embodiments provide the first metal product formed by any of thesemethods. Such a product may, for example, comprise one or more ofnanowires, nanocubes, nanorods, nanopyramids, or nanotubes. Suchnanowires may have an average diameter of about 30 to about 150 nm, orfrom about 30 to about 110 nm, or from about 80 to about 100 nm. Someembodiments provide one or more articles comprising at least one suchnanowire. Such articles may, for example, comprise electronic devices,transparent conductive films, and the like.

These embodiments and other variations and modifications may be betterunderstood from the brief description of figures, figures, description,exemplary embodiments, examples, and claims that follow.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows an embodiment of a reaction system with a continuous-flowtubular reactor.

FIG. 2 shows an embodiment of a reaction system with two continuous-flowtubular reactor stages and an inter-stage feed point.

FIG. 3 shows a micrograph of the product suspension of Example 1.

FIG. 4 shows a micrograph of the product suspension of Example 2.

FIG. 5 shows a micograph of the product suspension of ComparativeExample 3 after 1 hr at reaction temperature.

FIG. 6 shows a micograph of the product suspension of ComparativeExample 3 after 2 hrs at reaction temperature.

FIG. 7 shows a micograph of the product suspension of ComparativeExample 3 after 3 hrs at reaction temperature.

DESCRIPTION

All publications, patents, and patent documents referred to in thisdocument are incorporated by reference herein in their entirety, asthough individually incorporated by reference.

U.S. Provisional Application No. 61/442,874, filed Feb. 15, 2011,entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, ishereby incorporated by reference in its entirety.

U.S. patent application Ser. No. 13/347,986, filed Jan. 11, 2012,entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, ishereby incorporated by reference in its entirety.

Introduction

Silver nanowires (AgNW) are a unique and useful wire-like form of themetal in which the two short dimensions (the thickness dimensions) areless than 300 nm, while the third dimension (the length dimension) isgreater than 1 micron, preferably greater than 10 microns, and theaspect ratio (ratio of the length dimension to the larger of the twothickness dimensions) is greater than five. They are being examined asconductors in electronic devices or as elements in optical devices,among other possible uses.

A number of procedures have been presented for the preparation of AgNW.See, for example, Y. Xia, et al. (Angew. Chem. Int. Ed. 2009, 48, 60),which is hereby incorporated by reference in its entirety. These includethe “polyol” process, in which a silver salt is heated in a polyol(typically ethylene glycol (EG)) in the presence of polyvinylpyrrolidone(PVP), yielding a suspension of AgNW in EG, from which the wires can beisolated and/or purified as desired.

Y. Sun, B. Mayers, T. Herricks, and Y. Xia (Nano Letters, 2003, 3(7),955-960), hereby incorporated by reference in its entirety, propose thatAgNW are the result of the growth of multiply-twinned particles (MTP) ofsilver metal. P.-Y. Silvert et al. (J. Mater. Chem., 1996, 6(4), 573-577and J. Mater. Chem., 1997, 7, 293-299, both of which are herebyincorporated by reference in their entirety) describe the formation ofcolloidal silver dispersions in EG in the presence of PVP. Chen et al.(Nanotechnology, 2006, 7, 466-74), hereby incorporated by reference inits entirety, describe effects of changing seed concentrations onmorphology.

US patent publication 2010/0242679 and Japanese patent publication2010-255037 describe AgNW synthesis using continuous-flow stirred tankreactors.

Applicants have discovered that continuous-flow tubular reactors may beused to produce high aspect ratio AgNW with narrow nanowire lengthdistributions. Such tubular reactors can enable precise control oftemperature and reaction time without use of excessive agitation,thereby improving product uniformity.

FIG. 1 shows an embodiment of a reaction system with a continuous-flowtubular reactor. A feed pump [101] supplies raw materials, catalysts,and solvents to the continuous-flow tubular reactor [102], a portion ofwhich is contained in a thermostatted oven [103]. The downstream portionof the tubular reactor is immersed in a quench bath [104], with theproduct exiting the outlet of the reactor [105].

FIG. 2 shows an embodiment of a reaction system with two continuous-flowtubular reactor stages and an inter-stage feed point, where the feedpumps have been omitted from the figure for clarity. The first tubularreactor stage [201] may, for example, be used to prepare a seeddispersion, which is fed to the second reactor stage [202]. The otherraw materials, catalysts, and solvents may also be supplied to thesecond reactor stage at the inter-stage feed point [203].

Reducible Metal Ions and Metal Products

Some embodiments provide methods comprising reducing at least onereducible metal ion to at least one metal nanowire. A reducible metalion is a cation that is capable of being reduced to a metal under someset of reaction conditions. In such methods, the at least one firstreducible metal ion may, for example, comprise at least one coinagemetal ion. A coinage metal ion is an ion of one of the coinage metals,which include copper, silver, and gold. Or such a reducible metal ionmay, for example, comprise at least one ion of an IUPAC Group 11element. An exemplary reducible metal ion is a silver cation. Suchreducible metal ions may, in some cases, be provided as salts. Forexample, silver cations might, for example, be provided as silvernitrate.

Preparation Methods

A common method of preparing nanostructures, such as, for example,nanowires, is the “polyol” process. Such a process is described in, forexample, Angew. Chem. Int. Ed. 2009, 48, 60, Y. Xia, Y. Xiong, B. Lim,S. E. Skrabalak, which is hereby incorporated by reference in itsentirety. Such processes typically reduce a metal cation, such as, forexample, a silver cation, to the desired metal nanostructure product,such as, for example, a silver nanowire. Such a reduction may be carriedout in a reaction mixture that may, for example, comprise one or morepolyols, such as, for example, ethylene glycol (EG), propylene glycol,butanediol, glycerol, sugars, carbohydrates, and the like; one or moreprotecting agents, such as, for example, polyvinylpyrrolidinone (alsoknown as polyvinylpyrrolidone or PVP), other polar polymers orcopolymers, surfactants, acids, and the like; and one or more metalions. These and other components may be used in such reaction mixtures,as is known in the art. The reduction may, for example, be carried outat one or more temperatures from about 80° C. to about 190° C.

Metals, Metals Ions, Halides, and Metal Halides

In some embodiments, the reduction may be carried out in the presence ofone or more metals or metal ions (different from the at least onereducible metal ion), or in the presence of one or more halide ions, orboth. The metal ions used to catalyze wire formation are generallyprimarily reported to be provided as a metal halide salt, usually as ametal chloride, for example, FeCl₂ or CuCl₂. See, for example, J. Jiu,K. Murai, D. Kim, K. Kim, K. Suganuma, Mat. Chem. & Phys., 2009, 114,333, which refers to NaCl, CoCl₂, CuCl₂, NiCl₂ and ZnCl₂; Japanesepatent application publication JP2009155674, which describes SnCl₄; S.Nandikonda, “Microwave Assisted Synthesis of Silver Nanorods,” M. S.Thesis, Auburn University, Aug. 9, 2010, which refers to NaCl, KCl,MgCl₂, CaCl₂, MnCl₂, CuCl₂, and FeCl₃; S. Nandikonda and E. W. Davis,“Effects of Salt Selection on the Rapid Synthesis of Silver Nanowires,”Abstract INOR-299, 240th ACS National Meeting, Boston, Mass., Aug.22-27, 2010, which discloses NaCl, KCl, MgCl₂, CaCl₂, MnCl₂, CuCl₂,FeCl₃, Na₂S, and NaI; Chinese patent application publicationCN101934377, which discloses Mn²⁺; Y. C. Lu, K. S. Chou, Nanotech.,2010, 21, 215707, which discloses Pd²⁺; and Chinese patent applicationpublication CN102029400, which discloses NaCl, MnCl₂, and Na₂S. Use ofKBr has been disclosed in, for example, D. Chen et al., J. Mater. Sci.:Mater. Electron., 2011, 22(1), 6-13; L. Hu et al., ACS Nano, 2010, 4(5),2955-2963; and C. Chen et al, Nanotechnology, 2006, 17, 3933. Use ofNaBr has been disclosed in, for example, L. Zhou et al., Appl. Phys.Letters, 2009, 94, 153102. Japanese patent application publication2009-155674 discloses use of SnCl₄. U.S. patent application publication2010/0148132 discloses use of NaCl, KCl, CaCl₂, MgCl₂, and ZnCl₂. U.S.patent application publications 2008/0210052 and 2011/0048170 discloseuse of quaternary ammonium chlorides. See also Z. C. Li et al., Micro &Nano Letters, 2011, 6(2), 90-93; and B. J. Wiley et al., Langmuir, 2005,21, 8077. These and other compounds will be understood by those skilledin the art.

Continuous-Flow Reactors and Tubular Reactors

In at least some embodiments, at least one metal ion is reduced to atleast one metal in a continuous-flow reactor. In such a continuous-flowreactor, at least one feed composition or compositions (“feed”)comprising the at least one metal ion is supplied to the reactor and atleast one product composition or compositions (“product”) comprising theat least one metal is withdrawn from the reactor. The feed may, forexample, by supplied at a fixed flow rate, at a time varying flow rate,intermittently, and so on. The product may, for example, be withdrawn ata fixed flow rate, at a time varying flow rate, intermittently, and soon.

In such a continuous-flow reactor, at least some of the feed is suppliedto the reactor after at least some of the product is withdrawn from thereactor. This may be contrasted with a batch reactor, wheresubstantially all of the feed compositions comprising the at least onemetal ion are supplied to the reactor prior to or at the start of thereduction, and where substantially all of the product compositions arewithdrawn after the feed compositions are fed. And it may be contrastedwith a semi-batch reactor, where some of the feed compositions aresupplied prior to or at the start of the reduction and some of the feedcompositions are supplied thereafter, and where substantially all of theproduct compositions are withdrawn after the feed compositions are fed.

The temperature of the contents of a continuous-flow reactor may beuniform or may vary according to location or time. The pressure of thecontents of a continuous-flow reactor may be uniform or may varyaccording to location or time. The number of phases present in thecontinuous-flow reactor may be uniform or may vary according to locationor time.

In at least some embodiments, the reduction may be carried out in atleast one continuous-flow reactor comprising at least one tubularreactor. In such a tubular reactor, at least one feed composition orcompositions (“feed”) comprising the at least one metal ion is suppliedto one or more inlets to the reactor and at least one productcomposition or compositions (“product”) comprising the at least onemetal is withdrawn from one or more outlets of the reactor. The feedmay, for example, by supplied at a fixed flow rate, at a time varyingflow rate, intermittently, and so on. The product may, for example, bewithdrawn at a fixed flow rate, at a time varying flow rate,intermittently, and so on.

Such a tubular reactor may be contrasted with a stirred reactor, whichcomprises one or more rotating agitators to mix the reactor's contents.A tubular reactor will have at least one path between at least one inletand at least one outlet that does not contact such a rotating agitator.In some cases, all paths between inlets and outlets of the reactor willnot contact such a rotating agitator.

In at least some embodiments, such a tubular reactor may optionallycomprise one or more static mixing elements between at least some of itsinlets and outlets. Such static mixing elements may, in some cases,improve product homogeneity and increase heat transfer between thereactor contents and the walls of the reactor.

In at least some embodiments, such continuous-flow reactors may bearranged as parallel or series stages of reactors. The stages may, forexample, be stirred reactors, tubular reactors, or both. In such cases,feeds may be provided between at least some of the stages, or productsmay be withdrawn between at least some of the stages, or both. Otherdevices may optionally be provided between stages, such as, for example,devices for inter-stage heating or cooling of the material flowingthrough them.

In at least some embodiments, the feed composition comprises the atleast one reducible metal ion, at least one polyol, and at least one ofa protecting agent, a polar polymer, or a polar copolymer. In somecases, all of the components to be fed to the at least one continuousreactor may, for example, be combined to form a single feed mixture.Such an arrangement may, for example, provide improved productuniformity relative to that of a semi-batch reactor by reducing oreliminating variability due to changes in timing, quantities, and feedrates of the feeds to the semi-batch reactor.

In at least some embodiments, at least a portion of at least one of theproduct streams of a continuous-flow reactor may be provided to at leastone of the inlets of the same or a different continuous-flow reactorusing one or more recycle streams. Such a recycle stream may optionallycomprise one or more surge tanks or compartments to help manageinventories that are not in the reactor or reactors. These and othervariations will be understood by those skilled in the art.

Nanostructures, Nanostructures, and Nanowires

In some embodiments, the metal product formed by such methods is ananostructure, such as, for example, a one-dimensional nanostructure.Nanostructures are structures having at least one “nanoscale” dimensionless than 300 nm, and at least one other dimension being much largerthan the nanoscale dimension, such as, for example, at least about 10 orat least about 100 or at least about 200 or at least about 1000 timeslarger. Examples of such nanostructures are nanorods, nanowires,nanotubes, nanopyramids, nanoprisms, nanoplates, and the like.“One-dimensional” nanostructures have one dimension that is much largerthan the other two dimensions, such as, for example, at least about 10or at least about 100 or at least about 200 or at least about 1000 timeslarger.

Such one-dimensional nanostructures may, in some cases, comprisenanowires. Nanowires are one-dimensional nanostructures in which the twoshort dimensions (the thickness dimensions) are less than 300 nm,preferably less than 100 nm, while the third dimension (the lengthdimension) is greater than 1 micron, preferably greater than 10 microns,and the aspect ratio (ratio of the length dimension to the larger of thetwo thickness dimensions) is greater than five. Nanowires are beingemployed as conductors in electronic devices or as elements in opticaldevices, among other possible uses. Silver nanowires are preferred insome such applications.

Such methods may be used to prepare nanostructures other than nanowires,such as, for example, nanocubes, nanorods, nanopyramids, nanotubes, andthe like. Nanowires and other nanostructure products may be incorporatedinto articles, such as, for example, electronic displays, touch screens,portable telephones, cellular telephones, computer displays, laptopcomputers, tablet computers, point-of-purchase kiosks, music players,televisions, electronic games, electronic book readers, transparentelectrodes, solar cells, light emitting diodes, other electronicdevices, medical imaging devices, medical imaging media, and the like.

EXEMPLARY EMBODIMENTS

U.S. Provisional Application No. 61/442,874, filed Feb. 15, 2011,entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, whichis hereby incorporated by reference in its entirety, disclosed thefollowing 26 non-limiting exemplary embodiments:

-   A. A method comprising:    -   providing at least one first composition comprising at least one        first reducible metal ion; and    -   reducing the at least one first reducible metal ion to at least        one first metal in the presence of at least one first protecting        agent and at least one first solvent,    -   wherein the reduction is performed in at least one first        continuous-flow reactor comprising at least one tubular reactor.-   B. The method according to embodiment A, wherein the at least one    first reducible metal ion comprises at least one coinage metal ion.-   C. The method according to embodiment A, wherein the at least one    first reducible metal ion comprises at least one ion from IUPAC    Group 11.-   D. The method according to embodiment A, wherein the at least one    first reducible metal ion comprises at least one ion of silver.-   E. The method according to embodiment A, wherein the at least one    first compound comprises silver nitrate.-   F. The method according to embodiment A, wherein the reduction is    performed in the presence of at least one element from IUPAC Group 8    or IUPAC Group 14.-   G. The method according to embodiment A, wherein the reduction is    performed in the presence of iron or an ion of iron.-   H. The method according to embodiment A, wherein the wherein the    reduction is performed in the presence of tin or an ion of tin.-   J. The method according to embodiment A, wherein the reduction is    performed in the presence of at least one metal chloride.-   K. The method according to embodiment A, wherein the at least one    first protecting agent comprises at least one of: one or more    surfactants, one or more acids, or one or more polar solvents.-   L. The method according to embodiment A, wherein the at least one    first protecting agent comprises polyvinylpyrrolidinone.-   M. The method of embodiment A, wherein the at least one first    solvent comprises at least one polyol.-   N. The method of embodiment A, wherein the at least one first    solvent comprises at least one of: ethylene glycol, propylene    glycol, glycerol, one or more sugars, or one or more carbohydrates.-   P. The method of embodiment A, wherein the composition has a ratio    of the total moles of the at least one second metal or metal ion to    the moles of the at least one first reducible metal ion from about    0.0001 to about 0.1.-   Q. The method of embodiment A, wherein the reduction is carried out    at one or more temperatures from about 120° C. to about 190° C.-   R. The method of embodiment A, wherein the reduction is carried out    in the presence of at least one second composition comprising seed    particles.-   S. The method of embodiment R, wherein the second composition    comprises at least one coinage metal or coinage metal ion.-   T. The method according to embodiment R, wherein the at least one    second composition comprises at least one element from IUPAC Group    11.-   U. The method according to embodiment R, wherein the at least one    second composition comprises silver or an ion of silver.-   V. The method according to embodiment R, wherein the seed particles    are formed by a method comprising:    -   providing at least one third metal ion; and    -   contacting the at least one third metal ion with at least one        second protecting agent and at least one second solvent.-   W. The method according to embodiment V, wherein the seed particles    are formed in at least one second continuous-flow reactor.-   X. The method according to embodiment W, wherein the at least one    second continuous-flow reactor comprises at least one tubular    reactor.-   Y. At least one first metal product formed by the method of    embodiment A.-   Z. The product according to embodiment Y, comprising one or more of    nanowires, nanocubes, nanorods, nanopyramids, or nanotubes.-   AA. The product according to embodiment Y, comprising at least one    nanowire.-   AB. At least one article comprising at least one nanowire of    embodiment AA.

EXAMPLES Example 1

40 mL of a solution of 284.0 g polyvinylpyrrolidone (PVP, 55,000molecular weight) in 3 L ethylene glycol (EG), 40 mL of a solution of144.7 g AgNO₃ in 3 L of EG, 560 mL of EG, and 2.6 mL of a 6 mM solutionof FeCl₂ in EG were blended and charged to an addition funnel equippedto drip into a syringe body that fed the inlet of a peristaltic pump(MASTERFLEX® 7518-10 pump head equipped with 0.188 in ID/0.375 in ODflexible tubing and driven by a 6-to-600 RPM MASTERFLEX® 7521-40 ConsoleDrive). The outlet of the pump fed the inlet of a ca. 200 ft long run of0.25 in OD stainless-steel tubing (0.049 in wall thickness).Approximately 95% of the tubing was located in a BLUE M® oven, with thefinal 5% of the tubing being immersed in an ice water bath outside ofthe oven. The outlet of the tubing fed a product receiver.

The oven was heated to 144.5° C., after which the pump speed control wasset to deliver 11.9 mL/min and the addition funnel drip rate wasadjusted to maintain a constant head upstream of the pump. After 64 min,the pump speed control was increased to deliver 185 mL/min, with acompensating adjustment in the addition funnel drip rate. When abrownish grey suspension appeared on the outlet of the stainless steeltubing, the pump rate was decreased to deliver 11.9 mL/min, with acompensating adjustment in the addition funnel drip rate.

FIG. 3 is a micrograph of the product suspension, showing silvernanowires and many particles.

Example 2

40 mL of a solution of 284.0 g polyvinylpyrrolidone (PVP, 55,000molecular weight) in 3 L ethylene glycol (EG), 40 mL of a solution of144.7 g AgNO₃ in 3 L of EG, 560 mL of EG, and 2.6 mL of a 13.6 mMsolution of SnCl₂.2H₂O in EG were blended and charged to the additionfunnel of the apparatus of Experiment 1. The oven was heated to 165° C.,after which the pump speed control was set to deliver 11.9 mL/min andthe addition funnel drip rate was adjusted to maintain a constant headupstream of the pump. After 95 min, the oven temperature was decreasedto 145° C. A grey product suspension was collected from the outlet ofthe stainless steel tubing.

FIG. 4 is a micrograph of the product suspension, showing many ca. 20 nmlong silver nanowires, some shorter silver nanowires, and a fewparticles.

Example 3 (Comparative)

40 mL of a solution of 284.0 g polyvinylpyrrolidone (PVP, 55,000molecular weight) in 3 L ethylene glycol (EG), 40 mL of a solution of144.7 g AgNO₃ in 3 L of EG, 560 mL of EG, and 8 mg of SnCl₂.2H₂O in 2.6mL EG were blended and charged to a 1 L round-bottom flask. This mixturewas mechanically agitated at 100 rpm and heated to 165° C. over 59 min.The reaction mixture was held between 163° C. and 166° C. and sampledhourly after 1 hr, 2 hr, and 3 hr at temperature. Each of these 1 gsamples were examined microscopically at 500×. In each case, only a fewshort wires were observed visually and were not easily photographed.

In order to photograph these products, 3 drops of each were diluted with1 mL of acetone, centrifuged at 500 G for 30 min, the clear supernatantdecanted, and the residue dispersed in isopropanol by shaking. Thesedispersions were applied to glass slides and the liquid evaporated.Photo micrographs were taken of each of these treated glass slides, asshown in FIGS. 5, 6, and 7, showing microparticles with low aspectratios and very few nanowires.

It is surprising that a batch reactor supplied with the identical feedcomposition of Example 2 did not produce the same silver nanowireproduct as that of the continuous-flow reactor of Example 2.

The invention has been described in detail with particular reference toa presently preferred embodiment, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. The presently disclosed embodiments are thereforeconsidered in all respects to be illustrative and not restrictive. Thescope of the invention is indicated by the appended claims, and allchanges that come within the meaning and range of equivalents thereofare intended to be embraced therein.

What is claimed:
 1. A method comprising: feeding at least one firstcomposition comprising at least one reducible metal ion and at least oneprotecting agent to the contents of at least one continuous-flow reactorcomprising at least one tubular reactor; heating the at least one firstcomposition prior to reducing the at least one reducible metal ion to atleast one metal nanowire; and withdrawing at least one secondcomposition comprising the at least one metal nanowire from the contentsof the at least one continuous-flow reactor.
 2. The method according toclaim 1, wherein at least some of the withdrawing of the at least onesecond composition occurs before at least some of the feeding of the atleast one first composition.
 3. The method according to claim 1, whereinat least some of the withdrawing of the at least one second compositionoccurs simultaneously with at least some of the feeding of the at leastone first composition.
 4. The method according to claim 1, wherein theat least one continuous-flow reactor consists essentially of the atleast one tubular reactor.
 5. The method according to claim 1, whereinthe at least one first composition further comprises at least onepolyol.
 6. The method according to claim 1, wherein the at least onefirst reducible metal ion comprises at least one coinage metal ion or atleast one ion from IUPAC Group
 11. 7. The method according to claim 1,wherein the reduction is performed in the presence of at least onesecond ion or atom comprising at least one ion or atom from IUPAC Group8 or at least one ion or atom from IUPAC Group
 14. 8. The methodaccording to claim 1, wherein the reduction is performed in the presenceof at least one halide ion.
 9. The method according to claim 1, whereinthe at least one metal nanowire comprises a length of at least about 10μm.
 10. The metal nanowire produced according to the method of claim 1.11. The method according to claim 1, wherein the at least one protectingagent comprises at least one polar polymer or at least one polarcopolymer.
 12. The method according to claim 1, wherein the at least onereducible metal ion comprises at least one silver ion.
 13. The methodaccording to claim 7, wherein the at least one second ion or atomcomprises at least one tin ion or atom.
 14. The method according toclaim 1, further comprising heating the at least one first composition,wherein at least a portion of the heating the at least one firstcomposition occurs simultaneously with the reducing the at least onefirst reducible metal ion.